On Tue, 3 Nov 2020 12:58:22 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> This change turns the HashTable that JVMTI uses for object tagging into a
>> regular Hotspot hashtable - the one in hashtable.hpp with resizing and
>> rehashing. Instead of pointing directly to oops so that GC has to walk the
>> table to follow oops and then to rehash the table, this table points to
>> WeakHandle. GC walks the backing OopStorages concurrently.
>>
>> The hash function for the table is a hash of the lower 32 bits of the
>> address. A flag is set during GC (gc_notification if in a safepoint, and
>> through a call to JvmtiTagMap::needs_processing()) so that the table is
>> rehashed at the next use.
>>
>> The gc_notification mechanism of weak oop processing is used to notify Jvmti
>> to post ObjectFree events. In concurrent GCs there can be a window of time
>> between weak oop marking where the oop is unmarked, so dead (the phantom
>> load in peek returns NULL) but the gc_notification hasn't been done yet. In
>> this window, a heap walk or GetObjectsWithTags call would not find an object
>> before the ObjectFree event is posted. This is dealt with in two ways:
>>
>> 1. In the Heap walk, there's an unconditional table walk to post events if
>> events are needed to post.
>> 2. For GetObjectWithTags, if a dead oop is found in the table and posting is
>> required, we use the VM thread to post the event.
>>
>> Event posting cannot be done in a JavaThread because the posting needs to be
>> done while holding the table lock, so that the JvmtiEnv state doesn't change
>> before posting is done. ObjectFree callbacks are limited in what they can
>> do as per the JVMTI Specification. The allowed callbacks to the VM already
>> have code to allow NonJava threads.
>>
>> To avoid rehashing, I also tried to use object->identity_hash() but this
>> breaks because entries can be added to the table during heapwalk, where the
>> objects use marking. The starting markWord is saved and restored. Adding a
>> hashcode during this operation makes restoring the former markWord (locked,
>> inflated, etc) too complicated. Plus we don't want all these objects to
>> have hashcodes because locking operations after tagging would have to always
>> use inflated locks.
>>
>> Much of this change is to remove serial weak oop processing for the
>> weakProcessor, ZGC and Shenandoah. The GCs have been stress tested with
>> jvmti code.
>>
>> It has also been tested with tier1-6.
>>
>> Thank you to Stefan, Erik and Kim for their help with this change.
>
> Coleen Phillimore has updated the pull request incrementally with one
> additional commit since the last revision:
>
> More review comments from Stefan and ErikO
src/hotspot/share/gc/shared/weakProcessorPhases.hpp line 41:
> 39: class Iterator;
> 40:
> 41: typedef void (*Processor)(BoolObjectClosure*, OopClosure*);
I think this typedef is to support serial phases and that it is probably no
longer used.
src/hotspot/share/gc/shared/weakProcessorPhases.hpp line 50:
> 48: };
> 49:
> 50: typedef uint WeakProcessorPhase;
This was originally written with the idea that WeakProcessorPhases::Phase (and
WeakProcessorPhase) should be a scoped enum (but we didn't have that feature
yet). It's possible there are places that don't cope with a scoped enum, since
that feature wasn't available when the code was written, so there might have be
mistakes.
But because of that, I'd prefer to keep the WeakProcessorPhases::Phase type and
the existing definition of WeakProcessorPhase. Except this proposed change is
breaking that at least here:
src/hotspot/share/gc/shared/weakProcessor.inline.hpp
116 uint oopstorage_index = WeakProcessorPhases::oopstorage_index(phase);
117 StorageState* cur_state = _storage_states.par_state(oopstorage_index);
=>
103 StorageState* cur_state = _storage_states.par_state(phase);
I think eventually (as in some future RFE) this could all be collapsed to
something provided by OopStorageSet.
enum class : uint WeakProcessorPhase {};
ENUMERATOR_RANGE(WeakProcessorPhase,
static_cast<WeakProcessorPhase>(0),
static_cast<WeakProcessorPhase>(OopStorageSet::weak_count));
and replacing all uses of WeakProcessorPhases::Iterator with
EnumIterator<WeakProcessorPhase> (which involves more than a type alias).
Though it might be possible to go even further and eliminate
WeakProcessorPhases as a thing separate from OopStorageSet.
-------------
PR: https://git.openjdk.java.net/jdk/pull/967
